What are the three principle sources of internal heat of terrestrial planets? Astronomy Our Solar System The Planets 1 Answer Peter Jun 30, 2016 Pressure, friction, and radiation. Answer link Related questions What effect does Earth’s gravitational force have on objects of different masses? What solar system is Pluto in? Which planet in our solar system is most earth like? What is the closest planet to the sun? Why are the planets closest to the sun rocky? Why is earth closest to the sun in January? Why is Mars closest to the Earth in August? What is accretion, and how did it form the Earth? How does accretion explain planet formation? How does accretion form planets? See all questions in The Planets Impact of this question 7929 views around the world You can reuse this answer Creative Commons License